1. Field of the Invention
The present invention relates to transparent electroconductive windows comprising a non-electroconductive assembly element, an electroconductive circuit portion carried by said element and one or more bus bars, adapted to be coupled to a source of electric potential to provide an electrical current through said circuit portion that is capable of heating the window. While the window may comprise a single assembly element in the form of a sheet of rigid, transparent material, the present invention has special utility in laminated windows.
Transparent electroconductive windows have been used in aircraft particularly for the purpose of removing fog, mist or ice that forms on a surface of the windshield when the window is exposed to extremely low outside temperatures while the temperature inside the aircraft is relatively warm. Heated windows of this type are also finding use in other vehicles, such as railway cars, boats, and custom atuomobiles, in other window structures such as buildings, and in transparent windows for freezers and the like. Essentially, the window comprises a base or substrate or rigid transparent non-electroconductive material, such as glass or a well known plastic substitute for glass, that supports an electroconductive coating which may be either a matrix of thin wires or of thin strips of bus bar material or a continuous transparent electroconductive coating such as tin oxide and/or other metallic oxides or metals, which coating is in electroconductive contact with one or more bus bars. The latter are adapted to be connected to a source of potential whenever it is desired to apply electric current through the electroconductive coating so as to heat the window and dissipate a build-up of mist or ice that normally forms on a surface of the window under certain atmospheric conditions.
Since the transparent electroconductive coating is usually very thin and fragile, it has been the custom to protect it by laminating the coated substrate to additional sheets or layers of transparent material, using well known interlayer materials such as polyvinyl acetals, particularly plasticized polyvinyl butyral, an polyurethanes. Either or both outer plies may be of glass or a transparent plastic.
In order to supply electrical energy for powering the transparent electroconductive coating, it is necessary that the coating be connected to a bus bar, which serves as a distributor of electrical energy from a voltage source to the electroconductive circuit portion. The bus bar must distribute the electrical energy uniformly to provide uniform heating over the entire coated surface. In addition, the electroconductive coatings usually used cannot withstand localized high currents without damage to or even destruction of the laminated window. When the coating is marred locally, an area of high current density may develop, which results in localized high heating.
It has also been suggested to use various materials other than a rigid transparent sheet to support the electroconductive circuit portion. For example, the thermoplastic interlayer material which is used to adhere a glass sheet or a sheet of other rigid transparent material to another sheet may itself serve as a carrier for narrow electroconductive wire. Also, a flexible interlayer may serve as the carrier for a transparent electroconductive circuit portion in the form of a coating bonded thereto. However, it is usually necessary that the circuit portion that provides the heat for the window be protected from scratching and other causes of damage, and also that the bus bar to heating element contact be maintained adequately throughout the entire area of contact between the bus bar and the electroconductive circuit portion of the heating element.
Regardless of the nature of the support provided in a window for the electroconductive circuit portion, the need existed for a suitable bus bar that is capable of making adequate conact with the circuit portion and of preventing or minimizing the effect of hot spots on the application of an electrical potential to said circuit portion.